Abstract

Ian W. D. Dalziel & Luis H. Dalla Salda write: In their interesting paper on ‘Ordovician paleogeography of Siberia and adjacent continent’, Torsvik et al. 1995 comment on the hypothesis of possible Laurentia–Gondwana collision during the Ordovician put forward by Dalla Salda et al. (1992a, b) and amplified by Dalziel et al. 1994. Torsvik and his colleagues agree with us that the palaomagnetic data do permit our ‘alternative palaogeographic approach’ of juxtaposition and collision between the two continental masses in mid-Ordovician times as well as their ‘archetypal’ scenario with its 3000–4500 km of intervening oceanic lithosphere (Torsvik et al. 1995, p. 284 and figs 11d–f). We appreciate their taking time to analyse our approach, and would like to comment briefly on their objections to our model.

(1) Our ‘tight fit’ of Laurentia and Gondwana for mid-Ordovician times (Dalziel et al. 1994, fig. 3) is said to indicate ‘unabridged faunal exchange’ between the two continents that is not supported by the marine faunal data (Torsvik et al. 1995, p. 285). In fact we propose not just a tight fit, but Alpine-style continent-continent collision. The presence of a latitudinal mountain range thousands of kilometres long and of Alpine proportions, is hardly compatible with exchange of marine faunas between the juxtaposed continents.

(2) Faunas indicative of ‘deep margins’ in the early Ordovician are implied to contradict our model (p. 285). Yet India and Asia were separated by deep water until just prior to their collision, and the presently colliding Australia